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A RunInfo file.

This is a small textfile-backed data structure containing info
about a pants run, such as the timestamp, commit sha etc.

This supercedes the mostly unused BuildInfo functionality, which was
removed, to avoid confusion.

Auditors: markcc
Merge branch 'master' into new_reporting

Conflicts:
	src/python/twitter/common/threading/BUILD
	src/python/twitter/pants/base/run_info.py
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